1. Field of the Invention
The present invention relates to butterfly valves and, more particularly, to butterfly valves used for control applications.
2. Description of the Prior Art
Since their inception, butterfly valves have been used as closure-type devices for on-off control valves. In the past few decades, butterfly valves have been upgraded considerably and are rapidly gaining acceptance in the process control industry as the basic component in flow control systems. This wide acceptance of butterfly valves is primarily due to several beneficial features, such as low initial cost, space and weight savings, higher capacity than globe valves, excellent control characteristics and corrosion resistance.
Most early butterfly valve discs were formed from a flat disc-like member having two generally parallel sides. More sophisticated type valve discs were developed having surfaces tapered uniformly from an enlarged center portion to the edge portions thereof, with the shaft secured within a bore in the enlarged center portion.
The hydrodynamic forces acting on larger valve discs of the conventional configuration were generally high and therefore a relatively large actuator was required to position the valve disc and maintain stability for control operations. The velocity of the fluid flowing over the surfaces of the disc created a pressure differential between the leading and trailing halves of the disc and as a result, a considerable dynamic closing torque was exerted on the disc. The torque characteristics of the typical prior art butterfly valve disc severely limited its range of stable operation and most discs could not be operated beyond the 72.degree. open position in throttling applications. As a result, it was necessary to utilize oversized butterfly valves in order to obtain the desired flow capacity. The need for oversized valves and the large dynamic closing torque necessitated the use of larger actuators and larger sources of activating force, such as pressurized fluid. Thus, the prior art conventional type butterfly valves when used in control applications had restricted capacity, required large actuators and had poor overall total force characteristics leading to unstable regions of operation and torque reversals at open positions of less than full capacity.
In an attempt to overcome the difficulties experienced with the conventional butterfly valve disc, many variations of "low-torque" discs have been developed. Examples of such discs are described in the following United States patents: U.S. Pat. Nos. 3,485,476; 3,516,640; and 3,770,242. Most of these discs have extended the operational range of the butterfly valve to a full 90.degree. open position and therefore have increased the butterfly valve capacity. The so-called "low-torque" discs reduced unit "peak" dynamic unbalance and changed the "peak" position from 72.degree. open to more than 80.degree. open; however, for the most part this was done at the cost of capacity at intermediate positions. This resulted in a higher torque at the intermediate positions due to the higher pressure differential across the disc at these positions. As a result, most of these "low-torque" discs truly exhibited no increase in C.sub.v over conventional discs at the full open position and in some cases, it was actually lowered.
Many of the so-called "low-torque" discs used fins or flanges on the trailing edge to create a drag force to oppose the closing torque and this restricted capacity.